Over the last weeks, we have been contacted by several customers using a late 2013 iMac system with a fusion drive and one of our audio interfaces on Mavericks.
Usually, drop outs not only in Cubase but also on Youtube, iTunes etc. occur
We just received the confirmation from Apple that a fix for this issue - which btw is not limited to our applications and audio interfaces - needs to come from Apple with an update.
Unfortunately, the latest Mavericks 10.9.1 update does not address this. We hope to have a fix or at least more information on this topic in January.
There is an ongoing discussion on the Apple forum on this:
Here is a description taken from the Apple forum that could be a workaround for the time being. The key is to bypass the flash part of the fusion drive:
1) Create a bootable OS X external Disk, Boot from that and then install [your audio application] on it.... and use that for [...] any Audio work..for the time being.. until a fix is released by Apple for this issue. Make sure you do not access any data on the FD drive after booting from the external drive otherwise the issue will reappear. until you restart your Mac and reboot from the EXT drive.. You can however, boot from the FD when not doing Audio related work.
2) Split your Fusion drive into an SSD and a Mech drive (again for the time being) and then reassemble the FD after the fix has been issued.
3) Return your iMac for one with an SSD or mech drive inside (if you still can..)
4) Do nothing and wait until Apple releases a fix for the problem... though there is no official ETA for any such fix...